BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to optical systems and more particularly
to optical computing, information and communication systems and logic elements for
use therein which utilize the principle of cross-phase modulation (XPM).
[0002] When an intense ultrashort light pulse propagates through a non-linear material,
it temporally distorts the atomic and molecular configuration of the material. This
distortion of the non-linear material instantaneously results in a change in the refractive
index of the material. This change in the index of refraction is directly proportional
to the intensity of the propagating intense light pulse. The change in the refractive
index of the non-linear material, in turn, causes a phase change in the propagating
intense light pulse. The phase change causes a frequency sweep within the pulse envelope,
typically resulting in a blue shift at the tail end of the pulse and a red shift at
the front of the pulse. Typically, the effect is a spectral broadening of the pulse
resulting in the generation of a supercontinuum. This spectral effect on the propagating
intense light pulse is typically referred to as a self-phase modulation effect.
[0003] In addition to experiencing self-phase modulation, an intense light pulse propagating
through a non-linear material will typically undergo self-focusing, that is, a narrowing
of the cross-sectional diameter of the pulse. Self-focusing occurs because, typically,
the intensity of a pulse of light is greatest at its center and weakest at its outer
edges. Since n is directly proportional to the intensity of the pulse, the center
of the pulse causes a greater change in refractive index of the non-linear material
than the outer edges of the pulse. Consequently, the center of the pulse travels slower
than its outer edges, causing the outer edges to bend in towards the center of the
pulse. This effect causes the beam to focus.
[0004] In addition to experiencing self-phase modulation and self-focusing, an intense light
pulse propagating through a non-linear material may also be used to induce the phase
modulation of and/or the focusing of a co-propagating weak light pulse. These phenomena
are typically referred to as cross-phase modulation and induced focusing, respectively.
[0005] Cross-phase modulation may result in either frequency shifting (i.e., blue shifting
or red shifting) or spectral broadening (i.e., supercontinuum generation), the particular
effect depending on the relative times at which the weak pulse and the intense pulse
propagate through the non-linear material. For example, if the intense pulse has a
greater wavelength than the weak pulse, the intense pulse will travel faster through
the non-linear material. Therefore, if the intense and weak pulses are sent propagating
into the non-linear material at the same time, the weak pulse will be exposed predominately
to the change in refractive index caused by the tail end of the intense pulse. (This
is referred to commonly as tail walk-off). The result of tail walk-off is a blue shift
of the weak pulse. Analagously, if the weak pulse is sent propagating into the non-linear
material ahead of the intense pulse, the weak pulse will feel the effects of the refractive
index change due to the front end of the intense pulse (front walk-off). The result
of front walk-off is a shift of the weak pulse to the red. Finally, if the weak and
intense pulses are sent propagating into the non-linear material so that the weak
pulse is subjected to the changes in the refractive index caused by both the tail
end and the front end of the intense pulse (e.g. symmetric walk-off or no walk-off),
the weak pulse broadens spectrally to both the red and the blue.
[0006] Spectral changes arising from cross-phase modulation may lead to changes in the temporal
profile of the weak pulse when it propagates into a dispersive medium (i.e. an optical
fiber) or a dispersive optical component (i.e. a grating or a prism). For example,
if cross-phase modulation results in the spectral broadening of the weak pulse, a
further progagation of the weak pulse through a grating pair may slow down its re-shifted
frequencies (generated by XPM at the pulse front) with respect to its blues shifted
frequencies (generated by XPM at the pulse back), and consequently reduces the pulse
duration of the weak pulse.
[0007] Cross-phase modulation may also be used to change the spatial distribution of copropagating
weak pulses. This effect occurs when the intense pulse generates a spatially-dependent
non-linear refractive index. For example, a pump pulse with a Gaussian spatial distribution
of its intensity generates a higher refractive index on the propagation axis of the
weak pulse. As a consequence, the outer edges of the weak pulse bend in towards the
center of the pulse, and the weak pulse focuses.
[0008] As a term of art, cross-phase modulation is frequently used generically to refer
to both cross-phase modulation and induced focusing.
[0009] Non-linear materials are very well known in the art. Examples of non linear materials
are BK-7 glass, CdSe, liquid CS₂, NaCl crystal, doped glasses, semiconductor bulk
and quantum structures, microcrystalline semiconductor particles in glasses polydiacetylene
organic polymer and optical fibers.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to optical computing and communication systems
which rely on the phenomena of cross-phase modulation to alter and control, either
or simultaneously, the spectral, temporal or/and spatial properties of ultrashort
light pulses for processing of information with high speed (up to tens of terahertz
regime) repetition rates. The present invention is also directed to a method for altering
and controlling, either, or simultaneously, the spectral, temporal or/and spatial
properties of ultrashort light pulses using cross phase modulation.
[0011] One optical communication system for transmitting information, which is constructed
according to the teachings of the present invention and which involves frequency shifting
(i.e. altering the spectral properties) comprises means for generating a first beam
of laser light and a second beam of laser light, said first beam comprising a series
of ultrashort pulses of a first frequency, said second beam comprising a series of
ultrashort pulses of a second frequency, said pulses of said first beam being stronger
in intensity than said pulses of said second beam, means for modulating said pulses
in the first beam according to predetermined information, means for combining said
modulated first beam and second beam to form a third beam, a non-linear material disposed
along the path of said third beam for receiving said third beam and for producing
a fourth beam, said fourth beam including pulses of said first frequency from said
modulated first beam, pulses of said second frequency from said second beam, and pulses
of a third frequency, said pulses of said third frequency resulting from XPM produced
by copropagation of said first and second beams in said non-linear material, filter
means disposed along the path of said fourth beam for filtering out pulses of said
first frequency, a beamsplitter disposed along the path of said fourth beam on the
output side of said filter means for splitting light passed by said filter means into
a fifth beam and a sixth beam, filter means disposed along the path of said fifth
beam for transmitting only pulses of said second frequency, detector means for detecting
pulses passed by said filter means on the fifth beam, filter means disposed along
the path of said sixth beam for passing only pulses of said third frequency, and detector
means for detecting pulses passed by said filter means in the path of the sixth beam.
[0012] Another optical communication system for transmitting information, which is constructed
according to the teachings of the present invention and which involves modulating
the time duration and amplitude (i.e. the temporal properties) of ultrashort pulses
comprises means for generating a first and second beams of laser light, said first
beam comprising a series of ultrashort pulses of a first frequency bandwidth, said
second beam comprising a series of ultrashort pulses of a second frequency bandwidth,
said pulses of said first beam being stronger in intensity than said pulses of said
second beam, said pulses of said second beam having a peak intensity p1, means disposed
along the path of said first beam for modulating said pulses according to predetermined
information, means for combining said first beam and second beam to form a third beam,
a non-linear material disposed along the path of said third beam for receiving said
third beam and for producing a fourth beam, said fourth beam including pulses of said
first frequency bandwidth, pulses of said second frequency bandwidth and pulses of
a third frequency bandwidth, said pulses of said third frequency bandwidth also having
a peak intensity p1, said pulses of said third frequency bandwidth being a spectrally
broadened version of said first frequency bandwidth caused by cross-phase modulation,
filter means disposed along the path of said fourth beam for filtering out pulses
of said first frequency bandwidth, means disposed along the path of said fourth beam
for optically delaying longer light wavelengths relative to shorter light wavelengths
and for producing a fifth beam, whereby said pulses of said second frequency bandwidth
become temporally expanded and consequently less intense while said pulses of said
third frequency bandwidth become temporally compressed and consequently more intense,
and detector means disposed along the path of said fifth beam for measuring said pulses,
said detector means set at a intensity detection threshhold level equal to p1.
[0013] Another optical communications system for transmitting information, which is constructed
according to the teachings of the present invention and which involves controlling
the spatial properties of ultrashort pulses comprises means for generating a first
and second beams of laser light, said first beam comprising a series of ultrashort
pulses of a first frequency, said second beam comprising a series of ultrashort pulses
of a second frequency, said pulses of said first beam being greater in intensity relative
to said pulses of said second beam, means disposed along the path of said first beam
for splitting said first beam into third and fourth beams, means disposed along the
path of said third beam for masking a portion of said third beam, means disposed along
the path of said fourth beam for masking a portion of said fourth beam, the part of
the third beam which is masked being different from the part of the fourth beam which
is masked, means for modulating said third beam, means for modulating said fourth
beam, means for combining said second, third and fourth beams to form a fifth beam,
a non-linear material disposed along the path of said fifth beam for receiving said
fifth beam and outputting sixth, seventh and eighth beams, each travelling along a
different direction, the sixth beam containing pulses from said second beam, the seventh
beam resulting from XPM and containing pulses from said second and third beams and
the eighth beam resulting from XPM and containing pulses from said second and fourth
beams, means disposed along the paths of said seventh beam for detecting only pulses
from said third beam and means and means disposed along the paths of said eighth beam
for detecting only pulses from said fourth beam.
[0014] An optical AND gate which is constructed according to the teachings of the present
invention and which utilizes the principle of cross-phase modulation includes a beamsplitter
for combining a pair of beams of light, a delay line for delaying one of the pair
of beams so that the two beams overlap, a non-linear medium disposed along the path
of the combined beam and a filter for filtering out certain frequencies in the beam
passed through the non-linear medium.
[0015] An optical invertor which is constructed according to the teachings of the present
invention and which utilizes the principle of cross phase modulation includes means
for generating a light beam, a beamsplitter, a delay line, a non-linear medium and
a filter.
[0016] In other embodiments of the invention the intense and weak beams have different polarizations
rather than different frequencies.
[0017] It is an object of the present invention to provide an optical communication system
that makes use of the principle of cross-phase modulation.
[0018] It is an object of this invention to provide a method for controlling either the
spectral, temporal and spatial properties of ultrashort pulses that makes use of the
principle of cross phase modulation.
[0019] It is another object of the present invention to provide an optical communication
system utilizing an optical processor that has the has the capacity to process data
streams in the GHZ to terahertz range.
[0020] It is another object of the present invention to provide an optical computing system
utilizing an optical processor that has the has the capacity to process data streams
in the GHZ to terahertz range.
[0021] It is yet still another object of the present invention to provide a new and novel
optical processor.
[0022] It is a further object of this invention to provide a method for controlling the
spectral, temporal and spatial properties of ultrashort pulses that makes use of the
principle of cross phase modulation.
[0023] It is another object of this invention to provide an all optical system in which
a plurality of sub-systems using the principle of cross-phase modulation are cascaded
together.
[0024] It is still another object of this invention to provide mechanisms for a new and
novel ultrafast optical information processor.
[0025] Various other features, objects and advantages will appear from the description to
follow. In the description, reference is made to the accompanying drawings which form
a part thereof, and in which are shown by way of illustration, specific embodiments
for practicing the invention. These embodiments will be described in sufficient detail
to enable those skilled in the art to practice the invention, and it is to be understood
that other embodiments may be utilized and that structural changes may be made without
departing from the scope of the invention. The following detailed description is,
therefore, not to be taken in a limiting sense, and the scope of the present invention
is best defined by the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] In the drawings in which like reference numerals or characters represent like parts
and wherein:
Fig. 1 is a schematic representation of one embodiment of an optical system for transmitting
information which is constructed according to the teachings of the present invention;
Fig. 1(a) is an illustration of a modification of modulator 18 in Fig. 1;
Fig. 2 is a schematic representation of another embodiment of an optical system for
transmitting information which is constructed according to the teachings of the present
invention;
Fig. 2(a) is a diagram showing the initial (i.e. before passing through the non-linear
medium), expanded and compressed probe (i.e. weak) pulses in the Fig. 2 embodiment;
Fig. 3 is a schematic representation of a third embodiment of an optical system for
transmitting information which is constructed according to the teachings of the present
invention;
Fig. 4 is a graphic representation of the transmissivity of one of the masks in the
Fig. 3 optical system, the transmissiveness of the mask being selected to make the
intense beam profile triangular in shape;
Fig. 5 is an illustration showing how the profile of a pulse can be changed using
the mask designed as in Fig. 4
Fig. 6 is a schematic of an optical computing system which includes an AND gate according
to the teachings of this invention;
Fig. 7 is a truth table for the AND gate in Fig. 6.
Fig. 8 is a schematic of an optical computing system which includes an INVERTER according
to this invention;
Fig. 9 is a truth table for the INVERTER IN Fig. 8.
Fig. 10 is a schematic of a modification of the system of Fig. 1;
Fig. 11 is a schematic of a miniaturized system according to this invention; and
Fig. 12 is a schematic of another embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] Referring now to the drawings and more particularly to Fig. 1, there is illustrated
a schematic view of one embodiment of an optical system for transmitting information,
the optical system being constructed according to the teachings of the present invention
and represented generally by reference numeral 11.
[0028] Optical system 11, which utilizes the spectral effects of cross phase modulation,
includes an optical processor 12.
[0029] Processor 12 includes a laser system 13 which is used to produce an output beam 14-1.
Beam 14-1 includes intense ultrashort pulses at a first frequency F1 and weaker ultrashort
pulses at a second frequency F2. Laser system 13 may comprise a laser 13-1 and a second
harmonic generating crystal 13-2. Laser 13-1 may be a mode locked Nd:YAG laser which
is capable of emitting a beam of laser light having intense pulses at 1060 nm. Examples
of other lasers which may be used are a Ti:sapphire Laser, an Alexandrite laser, a
Forsterite laser, a laser diode, a dye laser or a free electron laser.
[0030] A dichroic beamsplitter 14-2 splits beam 14-1 into a first or pump beam 15, and a
second or probe beam 17. Beamsplitter 14-2 is designed to transmit 90% at frequency
F1 and 10% at frequency F2 and reflect 90% at frequency F2 and 10% at frequency F1.
A narrow band filter 14-3 removes all frequencies except F1 from pump beam 15 and
a narrow band filter 14-4 removes all frequencies except F2 from probe beam 17.
[0031] Processor 12 also includes means 18 disposed along the path of beam 15 for modulating
beam 15 in accordance with predetermined information. Means 18 may comprise an optical
Kerr shutter 19 which is designed to transmit the intense pulses of first beam 15
only when gating pulses, emitted from a laser 21, simultaneously arrive thereat. Thus,
by controlling the emissions of laser 21, such as by a computer 23, it becomes possible
through the operation of shutter 19 to effectively encode information obtained from
computer 23 into the intense pulses of beam 15. Instead of a Kerrshutter and a laser,
an electro-optical modulator 24-1 activated by a pulse generator 24-2 as shown in
Fig. 1(a) or a photoconducting switch could be employed.
[0032] Processor 12 further includes a beamsplitter 25 for combining beams 15 and 17 and
producing a third beam 27 containing both pulses at frequency F2 from beam 17 and
pulses at frequency F1 from beam 15. Beam 15 passes into beamsplitter 25 after it
is passed through an adjustable optical delay 24 while beam 17 passes directly into
beamsplitter 25. Mirrors 28-1 and 28-2 are used to change the direction of beam 17
while adjustable optical delay 24 is used to adjust the path length of beam 15 so
that the pulses in beam 15 and the pulses in beam 17 will overlap inside non-linear
material X
(3), identified by reference numeral 29. The length of material 29 is sufficient to produce
"walk-off". Material 29 may be for example glass or an organic substance. If desired,
the intensity modulating means 18 can be eliminated and the adjustable optical delay
24 used as a mechanism for temporally modulating the arrival time of the pulses in
beam 15. As will hereinafter be explained, non-linear material 29 is used to modulate
the frequency of the pulses at frequency F2 in beam 27 using the modulated copropagating
pulses at frequency F1. The output from non-linear material 29 is a fourth beam 31
containing pulses at first frequency F1, pulses at second frequency F2 and pulses
at a third frequency F3, the pulses at the third frequency F3 being pulses from the
second beam 17 that are frequency shifted (an amount f) as a result of propagating
through non-linear material 29 with pulses from beam 15.
[0033] More specifically, if a pulse in beam 17 copropagates through non-linear material
29 with a pulse from beam 15, the output will be the pulse from beam 15 and a pulse
corresponding to a pulse from beam 17 frequency shifted an amount that is proportional
to the peak power of the pulse from beam 15. On the other hand, if there is no pulse
from beam 15 then the pulse from beam 17 will pass through non-linear material undistorted.
[0034] Processor 12 also includes a filter 33, which is diposed along the path of beam 31.
Filter 33, is selected to block the transmission of the pulses at frequency F1.
[0035] System 11 further includes an optical transmission channel 34. A beamsplitter 35,
which is also disposed along the path of beam 31 at the output side of filter 33,
is used to split beam 31 into a fifth beam 37 and a sixth beam 39. A pair of filters
41 and 43 are disposed along the paths beams 37 and 39, respectively. Filter 41 is
selected to pass only pulses at frequency F2 while filter 43 is selected to pass only
pulses at frequency F3. A pair of photodiodes 45 and 47, are used to detect the light
passed by filters 41 and 43, respectively. Photodiodes 45 and 47 are both electrically
connected to a computer 49 which processes the signals received from each photodiode.
[0036] The operation of system 11 as a means for transmitting information is hereinafter
described. Laser 13 is activated, causing the emission of an output beam from which
is derived a first beam 15 of intense pulses at frequency F1 and a second beam of
weak pulses at frequency F2. The information to be transmitted is sent from computer
22 to laser 21 causing the emission therefrom of gating pulses. Those intense pulses
of beam 15 that arrive at shutter 19 at the same time that the gating pulses arrive
undergo a change in their polarization, permitting their transmission through shutter
19. All other intense pulses are blocked from passing through shutter 19. Consequently,
when beams 15 and 17 are combined into beam 27, there will usually be more weak pulses
than intense pulses. Those weak pulses that arrive at non-linear material 29 without
a corresponding intense pulse emerge from material 29 in beam 31 essentially unchanged.
In contrast, those weak pulses that arrive at non-linear material 29 at approximately
the same time as the intense pulses undergo cross-phase modulation and emerge from
material 29 in beam 31 at a different frequency, the particular shift in frequency
depending on whether the arrival of the weak pulses and the intense pulses is synchronized
to result in tail walk-off or front walk-off and the extent of the shift depending
on the relative intensity of the intense pulses. Beam 31, which contains pulses at
frequency F1 at frequency F2 and pulses at frequency F3 is passed through filter 33.
Filter 33 removes pulses at frequency F1. Beam 31 is then split into beams 37 and
39. Beam 37 is then passed through filter 41 which filters out the pulses at frequency
F2. The modulated pulses are then detected by photodiode 45, and the signal is sent
to computer 49. Beam 39 is passed through filter 43 which leaves only the non-modulated
pulses to be detected by photodiode 47 and processed by computer 49. As can be appreciated,
each signal received by photodiode 47 corresponds to a gating pulse whereas each signal
received by photodiode 45 corresponds to the absence of a gating pulses. In this manner,
binary information may be transmitted through system 11.
[0037] While, for the sake of convenience, source 13 has been represented as a single laser
system which simultaneously generates pulses at different frequencies and intensities,
it is to be understood that two separate lasers could easily be used if properly synchronized.
Also, it is to be understood that while system 11 is designed specifically to process
binary information, tertiary information or higher degrees of information could easily
be transmitted by increasing the number of differing intense pulses (and the associated
number of shutter mechanisms). It is also to be understood that shutter 19 could be
replaced by an electro-optic shutter. Also, shutter 19 and laser 21 could be replaced
by an electro-mechanical shutter. Furthermore, shutter 19 can be eliminated completely
by triggering the emission of intense pulses from laser source 13 with electrical
signals from computer 23.
[0038] Referring now to Fig. 2, there is shown another embodiment of an optical system for
transmitting information constructed according to the teachings of the present invention
and represented generally by reference numeral 51.
[0039] System 51, which utilizes the temporal effects of cross phase modulation to produce
a pulse compression type switch, includes a laser system 13 for generating a beam
14-1 of ultrashort laser light pulses, beam 14-1 including intense pulses of one frequency
F1 and weak pulses of another frequency F2, a dichroic beamsplitter 14-2, a pair of
filters 14-3 and 14-4, a pair of deflection mirrors 28-1 and 28-2, an adjustable optical
delay 24, a beamsplitter 25, modulating means 18, a nonlinear medium 30 and a filter
33, all arranged and functioning as in the Fig. 1 embodiment except that the length
of non-linear medium 30 is such that there is effectively no "walk-off".
[0040] System 51 also includes a pair of parallel grating plates 53 and 55, which receive
fourth beam 31 from filter 33 and produce a fifth beam 57. As will be explained later
in greater detail, grating plates 53 and 55 are used to temporally resolve fourth
beam 31 by optically delaying the longer wavelengths of light relative to the shorter
wavelengths of light. A photodiode 59 or other light sensitive measuring device is
disposed along the path of fifth beam 57. Finally, a computer 61 for processing the
signals emitted by photodiode 59 is electrically connected to photodiode 59. For reasons
to be discussed more fully below, computer 61 is programmed so that it will only process
signals having an intensity above a predetermined threshold, the threshold being for
example the intensity of weak pulses that pass through non-linear medium 29 without
corresponding intense pulses.
[0041] Before discussing the operation of system 51, it is important to understand that,
while for the sake of simplicity, the weak and intense pulses emitted from laser system
13 have been described as being of two different frequencies, the weak and intense
pulses are actually of two different frequency bandwidths, each frequency bandwidth
being for example a few tenths of a nanometer wide. Consequently, each pulse includes
frequency components from across its entire bandwidth. However, these frequency components,
while being spread over the spectral width of the pulse, are nonetheless homogeneously
distributed over the entire temporal width of the pulse. In other words, at any point
in time, the distribution of frequency components within each pulse is homogeneous.
[0042] With the above kept in mind, the description of the operation of system 51 is hereinafter
set forth. Laser system 13 is activated, causing the emission therefrom of laser light
including intense pulses of one frequency bandwidth within beam 15 and of weak pulses
of another frequency bandwidth within beam 17. Instead of a laser system comprising
single laser, two lasers could be employed, each emitting a separate beam. Information
from computer 23 is then encoded into beam 15 using shutter 19 in the manner described
above to eliminate certain intense pulses. Beams 15 and 17 are then combined using
mirrors 28-1 and 28-2 and beamsplitter 25 to produce beam 27. Beam 27 now consists
of weak pulses and intense pulses, synchronized using adjustable optical delay 24
so that they arrive simultaneously at non-linear material 30.
[0043] Beam 27 then travels across non-linear material 30, being transformed in the process
by cross-phase modulation into beam 31. Beam 31 includes intense pulses and two varieties
of weak pulses, namely, non-modulated weak pulses and modulated weak pulses. The non-modulated
weak pulses are those weak pulses that propagated across material 29 without a corresponding,
copropagating intense pulse. The non-modulated weak pulses are temporally and spectrally
indistinguishable from the weak pulses in beam 27. The modulated weak pulses are those
weak pulses that co-propagated through material 30 with intense pulses. The modulated
weak pulses have a spectrally broader bandwith than the weak pulses in beam 27. Moreover,
the modulated weak pulses are not spectrally homogeneous over time. Rather, the longer
wavelength components are more concentrated towards the temporal fronts of the pulses
and the shorter wavelength components are more concentrated towards the temporal tails
of the pulses.
[0044] After emerging from non-linear material 30, beam 31 is then passed through filter
33 which filters out the intense pulses. The non-modulated weak pulses and the modulated
weak pulses of beam 31 then arrive at grating plates 53 and 55. As discussed earlier,
plates 53 and 55 optically delay in time the longer wavelengths of each pulse relative
to the shorter wavelengths. This occurs because plate 53 disperses beam 31 into its
frequency components (the longer wavelengths being deflected at a greater angle than
the shorter wavelengths and, hence, traveling a greater distance to plate 55) while
plate 55 receives the components and recombines them to form beam 57. The resultant
effect of passing through grating plates 53 and 55 is as follows: For the weak non-modulated
pulses, each of which is homogeneous in frequency distribution, passage through the
plates results in temporal expansion. This occurs because the longer wavelengths slow
down and go to the back of the pulse while the shorter wavelengths speed up and go
to the front of the pulse. One consequence of temporal expansion is that the pulse
becomes less intense. This occurs because while the temporal width of the pulse has
increased, its energy has not. Consequently, the same amount of energy must be spread
over a greater period of time.
[0045] In contrast, for the weak modulated pulses, each of which has longer wavelengths
concentrated at the front of the pulse and shorter wavelength concentrated at the
tail of the pulse, passage through the plates results in temporal compression. This
occurs because the longer wavelengths at the front of the pulse are slowed down while
the shorter wavelengths at the tail of the pulse are accelerated. The consequence
of temporal compression is that the pulse becomes more intense. This occurs because
while the temporal width of the pulse has decreased, its energy has not. Consequently,
the same amount of energy must be spread over a shorter period of time.
[0046] Beam 57, including its compressed and expanded pulses, then arrives at photodiode
59. Both compressed signals and expanded signals trigger the emission of an electrical
signal from photodiode 59 to computer 61. Because computer 61 is programmed to ignore
signals of an intensity less than the modulated weak pulses in beam 27, only the compressed
(i.e. modulated) pulses register. Because the compressed pulses are related to the
intense pulses sent through shutter 19 which, in turn, correspond to the information
to be transmitted, system 51 can so be used to transmit information.
[0047] Fig. 2(a) shows the shapes of an initial (i.e. before passing through non-linear
medium 29) a compressed pulse and expanded probe pulse, the initial pulse being identified
by reference numeral, 70-0, the compressed pulse being identified by reference numeral
70-1 and the expanded pulse by reference numeral 70-2.
[0048] In addition to being used as a system for transmitting information, system 51 may
be used for intensity modulating pulses. In addition system 51 may be used as a pulse
compression device by removing shutter 19, laser source 21, and computer 23 or as
an pulse expansion device by removing shutter 19, source 21 and computer 23 and programming
computer 61 to detect only expanded pulses.
[0049] Those modifications discussed in conjunction with system 11 are also applicable to
system 51.
[0050] Instead of using a pair of gratings 53 and 55 for temporally resolving the fourth
beam 31, a sequence of prisms or any optical component (or components) or material
(i.e. optical fibers) which can produce by group-velocity dispersion the relative
delay between short and long wavelengths may be employed.
[0051] Referring now to Fig. 3. there is shown a third embodiment of an optical system for
transmitting information constructed according to the teachings of the present invention
and represented generally by reference numeral 71.
[0052] As will be more fully explained below, system 71 is designed to exploit the principle
of induced focusing and utilizes the spatial effects of cross-phase modulation to
deflect a beam of light. Self-focusing occurs when a Gaussian shaped beam of intense
light travels through a non-linear medium because the intensity of the beam across
its cross-section is much greater at its center than around its outer edges. Consequently,
the increase in the refractive index of the non-linear medium is also greatest in
the center of the beam and weakest around the outer edges. This causes the center
of the beam to move slower than the edges which, in turn, causes the edges to bend
in towards the center. As a result, the beam narrows in cross-sectional diameter (i.e.
focuses). Induced focusing is identical to self-focusing except that the change in
the refractive index is applied to a weak beam that is copropagating with the intense
beam.
[0053] As can be seen, system 71 has many of the same components as systems 11 and 51.
[0054] System 71 includes a processor 72. Processor 72 includes a laser system 13 for generating
a beam 14-1 of ultrafast laser light, a beamsplitter 14-2 for splitting beam 14-1
into a pair of beams 15 and 17 and a pair of filters 14-3 and 14-4 for filtering beam
15 to contain only intense pulses of at frequency F1 and beam 17 to contain only weak
pulses at another frequency F2. System 71 also includes a deflection mirror 72 for
deflecting beam 15, a beamsplitter 73 for splitting beam 15 into two beams 73-1 and
73-2, a pair of modulators 18, one for modulating beam 73-1 and the other for modulating
beam 73-2, a mask 73-4 disposed along the path of beam 73-1, a mask 73-5, disposed
along the path of beam 73-2, a pair of deflection mirrors 73-61 and 73-62, and a pair
of beamsplitters 73-8 and 73-9 for combining beams 73-1 and 73-2 with beam 17. Masks
73-4 and 73-5 are designed arranged to mask off different portions of their respective
beams. Beamsplitters 73-8 and 73-9 combine the portions of beams 73-1 and 73-2 passed
by their respective masks along the beams 15, identified by reference numerals 74-1
and 74-2 to produce a third beam 75.
[0055] Processor 72 further includes a non-linear material 29, which is disposed along the
path of beam 75 and optical delays 72-1 and 72-2. As will be described later in more
detail, non-linear material 29 receives beam 75 and produces a fourth beam 77, a fifth
beam 79 and a sixth beam 80, fifth beam 79 and sixth beam 80 being angularly deflected,
and by different amounts, relative to fourth beam 77. Filters 81, 82 and 83 are disposed
along the path of beams 79 and 80, respectively to filter out the intense pulses present
therein.
[0056] System 71 further includes photodiodes 85, 86 and 87 which are disposed further along
the paths of beams 77, 79, and 80 receive beams 77, 79 and 80 and output corresponding
electrical signals to a computer 91 for processing.
[0057] System 71 is operated first by activating laser 13, causing the emission therefrom
of a beam of laser light having intense (i.e. pump) pulses of one frequency F1 and
weak (i.e. probe) pulses of another frequency F2. The intense pulses are split into
two beams 73-1 and 73-2 and modulated in accordance with the information from their
respective modulators 18 to permit specific intense pulses to pass therethrough. Each
beam 73-1 and 73-2 is partially masked by its respective mask 73-4 and 73-5 and then
combined at beamsplitter 73-8 and 73-9, respectively with the weak pulses of beam
17 to produce a third beam 75. The intense pulses in beam 73-1 and 73-2 are synchronized
with their corresponding weak pulses from beam 17 by using the optical delays so that
they will arrive at non-linear material 29 at the proper time with the pulses of beam
17.
[0058] The propagation of beam 75 through non-linear material 29 results in the creation
of beams 77 and 79 and 80. Beam 77, which was not subjected to induced focusing, consists
of the weak pulses of beam 75 that traveled through non-linear material 29 without
copropagating with intense pulses. Beam 79, which is angularly deflected by an angle
A relative to beam 77 as a result of induced focusing, consists of the copropagating
weak and intense pulses, the intense pulses being from beam 73-1. The reason why beam
79 is angularly deflected, rather than being reduced in cross-sectional diameter (the
typical result of induced focusing), is that the masking of the intense pulses leads
to an asymmetrical change in the refractive index of the non-linear medium. Consequently,
this asymmetry causes the intense pulses (and their copropagating weak pulses) to
be deflected in the direction of the masked portion of the intense beam.
[0059] Beam 80, is angularly deflected by an angle B, which is different from angle A, relative
to beam 77. Beam 80 consists of copropogating weak and intense pulses, the intense
pulses being from beam 73-2.
[0060] Beams 77 and 80 are then passed through filters 81 and 82, which remove the intense
light pulses therefrom. The pulses in paths 77, 79 and 80 are converted into electrical
signals by photodiodes 85, 87 and 88 and sent to computer 89 for processing.
[0061] While the above discussion makes it appear that beams 79 and 80 are collimated, the
reality is that beams 79 and 80 actually emerge from non-linear medium 29 as diverging
cones. However, most of the energy is concentrated in a small angle, the deflection
angle. One way to eliminate the energy other than at this small angle is to make the
beam profile of the intense beam triangular in shape. This may be done by designing
masks 73-4 and 73-5 to mask off a part of the beam such that the intense beam profile
becomes triangular. This may be done by varying the transmissivity of masks 73-4 and
73-5 over their cross-sectional area. A graph of transmissivity vs. radius for such
a mask 73-4 is shown in Fig. 4. Fig. 5 shows the shape of a pulse without the mask
of Fig. 4 and with the mask of Fig. 4; the pulse being gaussian shaped without the
mask and triangularly shaped with the mask. Catastrophic self focusing and filament
generation can be eliminated if the nonlinear medium is thin enough.
[0062] As can be appreciated, in the absence of a pump pulse from either beam 73-1 or beam
73-2, weak beam 17 will pass through non-linear medium 29 and emerge undeflected as
beam 77. On the other hand, a pump pulse from beam 73-1 will cause a deflection of
the emerging beam to beam path 79 and or pump pulse from beam 73-2 will cause a deflection
of the emerging beam to beam path 80.
[0063] Also, apparatus 71 can be used, if desired, as a mechanism for altering the spatial
distribution of light in a weak beam; i.e. beam 17.
[0064] As can be appreciated, the system in Fig. 3 can be easily modified to include more
than two pump beams so as to be able to transmit more than two sources of information,
or if desired, can be modified so as to have only one pump beam for use in transmitting
information from a single source.
[0065] The modifications discussed in conjunction with system 11 are applicable to system
71.
[0066] Referring now to Fig. 6 there is shown an optical computing logic device system 91
constructed according to this invention, device 91 including an AND gate 93 which
operates using the principle of cross-phase modulation and a detector 95. AND gate
93 includes an adjustable optical delay 97, a beamsplitter 99, non-linear medium 101
and a filter 103.
[0067] AND gate 93 is used to perform an AND function on a first beam 105 of intense pulses
of one frequency f
p and a second beam 107 of weak pulses of another frequency fo. Delay 97, delays beam
107 as necessary, so that beams 105 and 107 overlap. Beamsplitter 97 combines beams
105 and 107 to form a third beam 109 which is passed through non-linear medium 101.
The output from non-linear medium 101 is a fourth beam 111 which may include pulses
of frequency fp, pulses of frequency fo and pulses of a frequency (fo + Δf), the pulses
having a frequency (fo + f) resulting from cross-phase modulation and where f is the
change in frequency resulting from cross-phase modulation. Filter 103 removes pulses
of frequency fp and pulses of frequency fo and allows only pulses of frequency (fo
+ Δf) to pass through. The light passed through filter 103, i.e. the pulses having
a frequency (fo + Δf) , is detected by detector 95. Detector 95 may be a photodiode.
[0068] AND gate 93 operates in the following manner. If there is a pulse from beam 105 and
there is no pulse in beam 107, there will be no output from filter 103. If there is
no pulse in beam 105 and there is a pulse in beam 107 there will be no output from
filter 103. If there is a pulse in beam 105 and a pulse in beam 107, then there will
be an output from filter 103, namely a pulse having a frequency (fo + Δf). A truth
table for AND gate 93 is shown in Fig. 7
[0069] Referring now to Fig. 8 there is shown an optical computing logic device 111 constructed
according to this invention. Logic device 111 includes an INVERTER 113 which operates
on the principle of cross phase modulation and a detector 115. INVERTER 113 includes
a laser 117 for generating a weak beam 119 of ultrashort light pulses of frequency
fo (i.e. probe pulses), an adjustable optical delay 118 a beamsplitter 121 for combining
beam 119 with a signal or input beam 123 which is to be inverted by INVERTER 113 to
form a third beam 125, input beam 123 being an intense beam of ultrashort pulses of
frequency fp, (i.e. pump pulses), a non-linear medium 127 disposed along the path
of beam 125, the light emerging from non-linear medium 127 including pulses of frequency
fo, pulses of frequency fp and pulses of frequency (fo + Δf) where Δf is the change
i.e. shift, in frequency fo as a result of cross-phase modulation and a filter 129
for removing pulses of frequency fp and pulses of frequency (fo + Δf) and allowing
pulses of frequency fo to pass.
[0070] INVERTER 113 operates as follows. Laser 117 is continuously outputting pulses fo.
If there is a pump pulse fp, there will be no output from filter 129, while if there
is no pump pulse fp there will be an output from filter 129, namely, a pulse having
a frequency fo. Thus, INVERTER 113 only provides an output in the absence of a pump
pulse. A truth table for INVERTER 113 is shown in Fig. 9.
[0071] As can be appreciated, other logic elements using XPM can also be formed.
[0072] The embodiments of the present invention are intended to be merely exemplary and
those skilled in the art shall be able to make numerous variations and modifications
to it without departing from the spirit of the present invention. For example, the
pump signals (in all embodiments) could be generated by all-optical processors and
the output signals could be used in cascade as basic units of all-optical processors.
Also, the optical processors may be miniaturized using diode laser technology and
integrated optics. Also, instead of different frequencies, the pump and probe signals
could have the same frequency but have different polarizations. All such variations
and modifications are intended to be within the scope of the present invention as
defined in the appended claims.
[0073] Referring now to Fig. 10 there is shown a system 131 which is similar to system 11
but wherein the pump and probe pulses have different polarizations rather than different
frequencies. System 131 includes a laser system 133 having a laser 13-1 and a polarizer
135. The output beam from laser system 133 is split by beamsplitter 14-2 into an intense
beam 15-1 and a weak beam 17-1. A quarter-wave plate 137 changes the polarization
of weak beam 17-1 and a polarizer (analyzer) 139 removes intense beam 15-1 from the
beam 31-1 emerging from non-linear medium 29.
[0074] All embodiments of this invention can be miniaturized using diode technology and
integrated optics.
[0075] Referring now to Fig. 11 there is shown a system 141 similar to system 11 in Fig.
1 but that has been miniaturized using diode laser technology and integrated optics.
System 141 includes a pair of diode lasers 143 and 145, a computer 147 an integrated
optics modulator 149, a waveguide 151, a multiplexer 153, a waveguide 154, a demultiplexer
155, a non-linear material 157, a filter 158 and a pair of detectors 159 and 161.
[0076] Referring now to Fig. 12 there is shown an example of a system 171 in the form of
an all-optical beam scanner remotely driven by an all optical processor 172. System
171 includes an INVERTOR 113 and AND gate 93 an optical amplifier 177, a frequency
modulator 179, an OTC 181, a filter 183, an optical amplifier 185 and a beam scanner
187. Beam scanner 187 includes a laser system 13, a delay 72-2, a mirror 189, a beamsplitter
191 and a non-linear medium 29.
1. A method of transferring information from a first beam of light pulses to a second
beam of light pulses the method being characterised by the step of copropagating said
two beams through a non-linear medium, whereby said second beam will be altered according
to the information on the first beam as a result of cross-phase modulation.
2. A method according to claim 1 characterised in that the first beam is an intense
beam and the second beam is a weak beam.
3. A method according to claim 1 or 2 characterised in that the first and second beams
differ in frequency.
4. A method according to claim 1 or 2 characterised in that the first and second beams
differ in bandwidth.
5. A method according to claim 1 or 2 characterised in that the first and second beams
vary in polarisation.
6. An optical communication system characterised by:
(a) means (13) for generating first (15) and second (17) beams of laser light, said
first beam comprising a series of pulses of a first frequency, said second beam comprising
a series of pulses of a second frequency, said pulses of said first beam being stronger
in intensity relative to said pulses of said second beam;
(b) means (18) disposed along the path of said first beam for modulating said pulses
in said first beam according to predetermined information:
(c) means (25) for combining said first beam and second beam to form a third beam
(27);
(d) a non-linear material (299 disposed along the path of said third beam for receiving
said third beams and producing a fourth beam (31), said fourth beam including pulses
of said first frequency, pulses of a third frequency the pulses of said third frequency
resulting from the spectral effects of cross-phase modulation;
(e) means (33) disposed along the path of said fourth beams for filtering out pulses
of said first frequency;
(f) means (35) for splitting said fourth beam into a fifth beam (37) and a sixth beam,
(g) means (47) for detecting pulses in said fifth beam of said second frequency; and
(h) means (45) for detecting pulses in said sixth beam of said third frequency.
7. An optical communication system characterised by:
(a) means (13) for generating first (15) and second (17) beams of a laser light, said
first beam comprising a series of pulses of a frist frequency bandwidh, said second
beam comprising a series of pulses of a second frequency bandwidth, said pulses of
said first beam being stronger in intensity relative to said pulses of said second
beam, said pulses of said second beam having a peak intensity p1;
(b) means (18) disposed along the path of said first beam for modulating said pulses
according to predetermined information;
(c) means (25) for combining said first beam and second beam to form a third beam
(27);
(d) a non-linear material (30) disposed along the path of said third beam for receiving
said third beam and for producing a fourth beam (31), said fourth beam including pulses
of said first frequency bandwidth, pulses of said second frequency bandwidth, and
pulses of a third frequency bandwidth, said pulses of said third frequency bandwidth
also having a peak intensity p1, said pulses of third frequency bandwidth being a
spectrally broadened version of said pulses of said second frequency bandwidth caused
by cross-phase modulation;
(e) filter means (33) disposed along the path of said fourth beam for filtering out
pulses of said first frequency bandwidth;
(f) means (53, 55) disposed along the path of said fourth beam for temporally resolving
said pulses of said second and third frequency bandwidths and producing a fifth beam
whereby said pulses of said second frequency bandwidth become temporally expanded
and les intense while said pulses of said third frequency bandwidth become temporally
compressed and more intense; and
(g) means (59) disposed along the path of said fifth beam for measuring said pulses,
said measuring means being set at an intensity detection threshold equal to p1.
8. An optical communication system characterised by:
(a) means (13) for generating first and second beams of laser light, said first beam
comprising a series of pulses of a first frequency, said second beam comprising a
series of pulses of a of a second frequency, said pulses of said first beam being
greater in intensity relative to said pulses of said second beam;
(b) means (73) for splitting the first beam into third and fourth beams,
(c) means (18) disposed along the path of said third beam for modulating said pulses;
(d) means (73.4) disposed along the path of said third beam for masking a portion
of the cross-section of said third beam;
(e) means (18) disposed along the path of the fourth beam for modulating said pulses
of the cross section of the fourth beam;
(f) means (73.5) disposed along the path fo the fourth beam for masking a portion
of the cross section of the fourth beam, said masked portion being different from
sad masked portion of third beam,
(g) means (78.8,73.9) for combining said first , third and fourth beams to form a
fifth beam;
(h) a non-linear material (29) disposed along the path of said fifth beam for receiving
said fifth beam whereby a sixth beam and a seventh beam are outputted from said non-linear
material, said seventh beam travelling at a different angel relative to said sixth
beam;
(i) filter (18,829 disposed along the paths of said sixth beam and seventh beam for
filtering out pulese of said second frequency; and
(j) means (85, 87) disposed along the paths of said sixth beam and said seventh beam
for detecting said pulses of said second frequency.
9. The optical communication system according to claim 6,7 or 8 characterised in that
said modulating means includes a Kerr shutter (19), a laser (21) for gating said Kerr
shutter, and means (23) for triggering the emission of said laser source in accordance
with the information to be transmitted.
10. The optical communication of system claim 6 characterised the means for detecting
pulses in the fifth beam and the means for detecting pulses in the sixth beam each
comprises a photodiode .
11. The optical communication system of claim 6,7 or 8 and wherein the means for generating
first and second beams of light comprises a laser system (13), a beamsplitter (14.2)
and a pair of filters (14.3, 14.4).
12. The optical communication system of claim 6,7 or 8 and wherein the laser system
comprises a laser and a second harmonic generating crystal.
13. The optical communication system of claim 6,7 or 8 and further including an optical
delay (21) disposed along the path of one of the beams.
14. The optical communication system of claim 6 and further including an optical transmission
channel (34) between the non-linear material (29) and the means (35) for splitting
the fourth beam into fifth and sixth beams.
15. The optical system of claim 7 characterised in that the temporally resolving means
comprises a pair of parallel grating (55,53).
16. The optical system described in claim 8 and further comprising means for making
the intensity profile of said third and fourth beams triangular in shape.
17. A method of temporally compressing a weak pulse of light characterised by the
steps of:
(a) providing an intense pulse of a light that is different from the weak pulse;
(b) combining the two pulses of light to produce a third pulse of light;
(c) passing the third pulse of light through a non-linear material to produce a fourth
pulse of light, said fourth pulse of light being a spectrally broadened version of
said weak pulse produced by cross-phase modulation (XPM); and
(d) delaying the short light wavelengths in said fourth pulse relative to the longer
light wavelengths therewithin to produce a temporally compressed pulse.
18. an optical computing logic element operating acccroding to the method of any of
claims 1 to 5 characterised by;
(a) at least two sources of light pulses having different characteristics;
(b) means for selectively gating at least one of the light sources;
(c) means for combining the light pulses;
(d) a non-linear material through which the combined light pulses pass; and
(e) means for filtering out pulses having a characteristic produced in dependence
on the logic function.
19. An optical computing logic element characterised by an optical AND gate according
to claim 15.
20. An optical computing logic element characterised by an optical INVERTER according
to claim 15.
21. An optical AND gate according to claim 19 for performing and AND function on a
first beam of light having intense pulses of one frequency and a second beam of light
having weak pulses of light a second frequency characterised by:
(a) means (109) for combining the two beams to form a third beam;
(b) a non-linear medium (101) disposed along the path of the third beam, the light
passed through the non-linear medium comprising a fourth beam, the fourth beam having
pulses of the frequency of the first beam, the frequency of the second beam and pulses
of a third frequency;
(c) filter means (103) for removing pulses of the first frequency and either pulses
of the second or pulses of the third frequency.
22. An optical INVERTER according to claim 20 for inverting a first beam of light
of weak pulses of a first frequency characterised by;
(a) a second beam of light of intense pulse of a second frequency,
(b) means (121) for combining the first and second beams of light to form a third
beam,
(c) a non-linear medium (127) disposed along the path of the third beam; and
(d) a filter (129) for removing all light pulses passed by the non-linear medium other
than those of said first frequency.
23. A method of shifting the frequency of the pulses in a beam of ultrashort light
pulses the pulses having a frequency fo, the method characterised by the steps of:
(a) providing a non linear medium;
(b) copropagating the beam through the non-linear medium with a beam of ultrashort
light pulses of greater intensity and a frequency fp so as to produce by cross-phase modulation (XPM) pulses having a frequency fo + f,
wherein f is a change in frequency; and
(c) removing pulses of a frequency fp and fo fromt he beam emerging from the non linear
medium.
24. A method of angularly deflecting the pulses in a beam of ultrashort light pulses
the pulses having a frequency fo, the method characterised by the steps of:
(a) providing a non linear medium;
(b) copropagating the beam through the non-linear medium with a beam of ultrashort
light pulses of greater intensity and a frequency fp and which is partially masked,
wherein the intense beam will produce a spatial index of refraction change in the
weak beam as a result of cross-phase modulation.
25. A method of intensity modulating a beam of weak pulses of light the method characterised
by the steps of:
(a) providing a beam of intense pulses of a light that is different from the beam
of weak pulses;
(b) combining the two beams of light to produce a third beam of light;
(c) passing the third beam of light through a non-linear material to produce a fourth
beam of light the fourth beam of light including intese pulses from said intense beam
and pulses resulting from cross-phase modulation of said intense and weak beams;
(d) removing said intense pulses from the fourth beam, and then
(e) temporally resolving the remaining pulses in said fourth beam.
26. A method of altering the spatial distribution of light in a beam of ultrashort
weak light pulses the method characterised by the steps of:
(a) providing a non -linear medium; and
(b) copropagating the beam through the non-linear medium with a beam of ultrashort
light pulses of greater intensity, the second beam being different from the first
beam whereby the beam of grater intensity will alter the spatial distribution of the
light pulses in the beam of ultrashort weak pulses.
27. A method of processing information at GHZ to terahertz rates characterised by
the steps of:
(a) providing a non-linear medium having femtosecond or pisosecond response time,
such as an organic polymer material or an optical fiber made of fused silica, polymer
or glass; and
(b) copropagating through the non-linear medium a weak beam of ultrashort light pulses
and an intense beam of ultrashort light pulses, the intense beam containing the information
to be processed.
28. An optical communication system characterised by:
(a) means for generating first and second beams of polarised laser light, the polarisation
of the first beam being different from the polarisation of the second beam, said pulses
of said first beam being stronger in intensity relative to said pulses of said second
beam;
(b) means disposed along the path of said first beam for modulating said pulses in
said first beam according to predetermined information;
(c) means for combining said first beam and second beam to form a third beam;
(d) a non-linear material disposed along the path of said third beam for receiving
said third beam and producing a fourth beam, said fourth beam including pulses of
said first beam, pulses of said second beam and pulses corresponding to pulses of
said second beam but shifted in frequency as a result of cross-phase modulation (XPM);
(e) means disposed along the path of said fourth beam for filtering out pulses of
said first beam;
(f) means for splitting said fourth beam into a fifth beam and a sixth beam;
(g) means for detecting pulses in said fifth beam corresponding to said second beam;
and
(h) means for detecting pulses in said sixth beam corresponding to said second beam
but shifted in frequency.
29. A miniaturized processing system characterised by:
(a) means (143,145) for generating first and second beams of laser light;
(b) a non-linear material (157);
(c) waveguide means (51) coupling the outputs of the laser generating means to the
non-linear medium;
(d) modulating means (149) for modulating the output of the laser generating means;
(e) a pair of detectors (159, 161); and
(f) waveguide means (154) coupling the non-linear material to the detectors.
30. An all optical beam scanner operating in accordance with the method of claim 1
characterised by:
(a) means (51,52) for generating a pair of beams of ultrashort light pulses;
(b) an optical inverter (113) for performing an inverting function on one of the beams;
(c) an optical AND gate (939 for performing an AND function on the two beams,
(d) optical amplifier means (177) for amplifying the output of the AND gate;
(e) a frequency modulator (179) for modulating the output of the AND gate; and
(f) a beam scanner (187) utilizing cross-phase modulation.